1. Field of the Invention
The present invention relates to electronic circuitry, and, more particularly, to isolation and immunization of quiet circuitry from noise generated by noisy circuitry.
2. Description of the Related Art
Electronic circuit noise consists of undesirable signals which obscure desired circuit signals. Signals of interest in electronic circuits are often mixed with noise. Noise includes "random" noise of physical (often thermal) origin. For example, Johnson noise voltage is intrinsically generated by all resistors. A resistor may also include excess noise due to resistance fluctuations that produce a noise voltage in addition to the Johnson noise voltage. Such resistance fluctuations can differ based on the quality of the resistor.
Noise can also be another signal causing interference with the desired signal. Interfering signals can enter an electronic instrument through the power-line inputs or through signal input and output lines. In addition, signals can be capacitively coupled onto wires in the circuit, magnetically coupled to closed loops in the circuit, or electromagnetically coupled to wires acting as small antennas for electromagnetic radiation. Any of these can become a mechanism for coupling signals from one part of a circuit to another. Also, signal currents from one part of the circuit can couple to other parts through voltage drops on power supply lines.
Circuit noise limits the performance of electronic circuitry. Consequently, the suppression of the effects of noise by either isolation of circuit noise or immunization of circuits against the effects of noise is an important consideration in circuit and package design. The effect of noise in a circuit may be reduced by the use of various design techniques to reduce the noise generated by certain devices or circuits. The effect of noise in a circuit may also be reduced by the use of various layout techniques to isolate (e.g., with guard rings or shields) the sensitive circuits in question. Additionally, or alternatively, sensitive circuits may be immunized or made more tolerant to noise (e.g., by using differential circuitry). In difficult cases the solution may involve a combination of filtration of input and output lines, careful layout and grounding, and extensive electrostatic and magnetic shielding. Examples of noise suppression, isolation and immunization techniques may be found in N. Verghese, T. Schmerbeck, D. Allstot, Simulation Techniques and Solutions for Mixed-Signal Coupling in Integrated Circuits 235-253 (Kluwer Academic Publishers 1995) and in P. Horowitz, W. Hill, The Art of Electronics, pp. 430-466 (Cambridge University Press 1989).
Some types of noise, such as thermal noise in resistors and shot noise in transistors, is intrinsic to circuits including those elements. In mixed signal applications, however, digital switching and output driver noise often dominate such intrinsic noise. Digital switching noise reaches sensitive circuits via the substrate or via source or ground lines. Some systems are especially susceptible to digital switching noise. For example, communication applications (e.g., Ethernet or fiber channel) are especially susceptible to such noise, but analog-to-digital conversion, digital-to-analog conversion and analog audio processing applications (especially with switch capacitor filters) are also susceptible to such noise. Communications systems often include a sensitive clock recovery circuit or clock generation circuit that has tight jitter tolerances and the jitter may have a large dependence on noise that is on a power rail or in the substrate. Consequently, an inexpensive, integrated method of digital noise effect suppression or immunization is desired for analog sections of mixed signal applications.